COVID-19 Testing: The Basics

Please note, New South Family Medicine is NOT testing the public for COVID-19

 

Hey Fort Mill, SC!  We hope all of you are staying safe out there!

There has been so much talk about testing for COVID-19.  What does it all mean?  What is the test that is by mouth or nose?  What are these blood tests?  Titers??  It’s enough to make your head spin, so I thought I would break it down for you!

  1.  PCR swab:  The first test we started working with is called a PCR test.  This is the test where we collect specimens from deep in your nose.  Even that was controversial in the beginning of collection.  PCR stands for polymerase chain reaction.  The lab takes the specimen and amplifies it by millions. If the virus is present in the sample given, it is positive.   If there is no viral material in the sample, the test is negative.

So….this test directly tests for the virus.  NOT our antibody response to it.  What are the limitations of this test?  First, if the person is not yet shedding viral material, meaning if they are only a couple days into their illness, there might not be anything on the actual sample to amplify.  This is why we wait for about 4 days into someone’s illness to test – without a fever, we know the body hasn’t yet mounted an immune response and there may not yet be enough of the virus present to get a good sample (even if they have the illness and don’t know it yet).  Second, if the sample is not properly obtained (bad sample, wrong testing tube, not deep enough back there).  Both of these situations would also cause a FALSE NEGATIVE.

So why did these take so long to get back?  Backlog.  Our labs were so overwhelmed with the number of tests coming in, they had to prioritize the sickest patients first then test the rest.  Its taking about 7-10 days as the number of cases in NC/SC rise.  There are many different testing sites in the area that now have access to different labs that aren’t quite so backed up.

2.  Rapid antigen tests:  These are testing for a specific part of the virus. Results come back in about 30 minutes using a rapid machine, but these tests are only 80% sensitive, which means they miss 20% of infections.  That means that, for every 5 people tested this way alone, 1 will test negative when, in fact, they have COVID-19.  We ALWAYS recommend that, if your rapid antigen test is negative, it is followed with PCR swab.  If your rapid antigen test is positive, head home and quarantine because that is 99% accurate.

3.  Antibody blood testing:  This is a blood sample to see you if you may have been exposed to COVID-19 in the past.  This test should NOT be used to diagnose an acute infection.  Antibodies take about 4-6 weeks to mount against COVID-19, so this test will be negative in early infections.    We also don’t know how long immunity lasts for COVID-19.  It could be a few weeks, a few month, or a few years.  This virus is new and we just don’t have long-term data on that, so please do not assume, if your antibody test is positive, you are immune and safe.

Many states are requiring negative tests to enter.  Usually, this means a negative PCR swab, and still has limitations because they are taking days to come back.  Again, if tested too early, it can be falsely negative as well.  Not an easy answer, I know.

Yikes!  I hope I haven’t confused you and that this helps put these tests into perspective.  None of them are perfect, but collectively they give us information on the spread of the virus and how we respond to it.

Dr. Jessica